1. Field of the Invention
The present invention relates to an electron emitting device for use in a display apparatus, an electron beam drawing apparatus, a vacuum tube, an electron beam printer or the like, and more particularly to a semiconductor electron emitting device for inducing an avalanche amplification thereby emitting hot electrons to the outside, and an electron emitting device having a surface with negative electron affinity.
2. Related Background Art
Among the semiconductor electron emitting devices utilizing avalanche amplification, there is a known device having a junction of a p-type semiconductor and an n-type semiconductor on a semiconductor substrate (p-n junction device), and a device having a Schottky junction of a semiconductor layer and a metal or a metal compound (Shottky junction device).
The semiconductor electron emitting device of pn junction type utilizing avalanche amplification is for example disclosed in the U.S. Pat. Nos. 4,259,678 and 4,303,930.
In said semiconductor electron emitting device, a p-semiconductor layer and a n-semiconductor layer are formed on a semiconductor substrate, and a metal such as cesium is attached on the surface of said n-semiconductor layer to form an electron emitting part. An electron avalanche is induced by applying an inverse bias voltage to a diode formed by said p-and n-semiconductor layers to generate hot electrons, thereby emitting the electrons from said electron emitting part.
Also in said semiconductor emitting devices of Shottky junction type utilizing avalanche amplification, there is a known device inducing an avalanche amplification by applying an inverse bias voltage to a junction of a p-semiconductor layer and a metal electrode to generate hot electrons, thereby emitting the electrons from an electron emitting part.
However, in order to obtain a high electron emission current from such semiconductor electron emitting device utilizing avalanche amplification, it is generally required to supply a very high current to the device. In general there is required a current density of 10,000 .ANG. or higher in order to induce electron emission from the pn junction as explained above.
Such large current supply to the conventional semiconductor electron emitting device generates heat therein, giving rise to drawbacks such as unstable electron emitting characteristics or shortened service life of the device.
Consequently it is desirable to have an electron emitting device with reduced local heat generation.
Also in the conventional structures of the pn junction type explained above, a material of low work function is employed for reducing the work function of the electron emitting part, thereby lowering the inverse bias voltage.
A material of low work function, such as cesium, is conventionally employed for realizing electron emission without an excessively high inverse bias voltage, but such material, being chemically active and subject to the influence of local heat generation in the semiconductor layer, is unable to ensure stable operation. For this reason it is desirable to have an electron emitting device allowing the use of a relatively stable material for the purpose of reducing the work function.
Also for the electrode of the conventional electron emitting device of Shottky junction type, it is desirable to have a material capable of forming a Shottky junction that provides a low work function. However, in the conventional electron emitting devices, the freedom of selection of the electrode material has been limited because of a tendency of migration of the electrode material by local heat generation in the semiconductor layer and because of a large energy band gap of the semiconductor, so that the material selection for improving the device stability cannot be achieved in satisfactory manner. Also drawbacks result as in the conventional pn-junction type explained above if a cesium or cesium oxide layer is formed on the electron emitting part in order to reduce work function thereof.
For this reason it is desirable to have an electron emitting device enabling a wider selection of the electrode material for the Shottky electrode, with a low local heat generation.
On the other hand, semiconductor electron emitting devices utilizing a negative electron affinity (NEA) are disclosed for example in the Japanese Patent Publication Nos. 54-30274 and 60-25858.
In such electron emitting device, a n-semiconductor layer and a p-semiconductor layer are formed on a semiconductor substrate and a layer of a material of low work function such as cesium is formed on the surface of said p-semiconductor layer to reduce the work function at the surface, thereby forming an electron emitting part of a negative electron affinity state. In such device, a forward bias voltage is applied to a diode formed by said n- and p-semiconductor layers, thereby supplying the p-semiconductor layer with electrons and emitting electrons from the electron emitting part.
As explained above, the conventional forward-bias electron emitting device utilizing negative electron affinity requires, for inducing electron emission with a forward bias, a layer of a material of low work function, thereby forming a surface with a negative electron affinity.
Said low work function material has been composed for, example, of cesium in consideration of the energy band gap of the semiconductor material.
Also in such conventional semiconductor electron emitting device employing a work function reducing material, since the electrons in the n-semiconductor layer can reach the electron emitting part through the p-semiconductor layer under a forward biasing, it is necessary, for improving the electron emitting efficiency, to reduce the number of holes in the p-semiconductor layer and to reduce the thickness of said p-semiconductor layer in order to protect the electrons injected into said p-semiconductor layer from recombination with holes therein or from phonon scattering, but there are the following encountered drawbacks in such case.
More specifically, an increased resistance of the p-semiconductor layer leads to local heat generation therein. In order to obtain a high electron emission current from the semiconductor electron emitting device, it is generally required to supply a very high current to said device, and such current supply induces heat generation in the device. Such heat generation tends to cause evaporation or migration of the material of low work function which is generally not stable, such as cesium, thereby causing unevenness in the electron emitting area, unstable electron emitting characteristics and shortened service life of the device.
Furthermore, since cesium is chemically extremely active, stable operation can only be expected at a pressure of 10.sup.-7 Torr or lower, and the service life and the efficiency of the device are dependent on the level of vacuum.
For this reason it is desirable to have an electron emitting device with a surface of negative electron affinity state without relying on a work function reducing material, or a device allowing the use of a relative stable work function reducing material, instead of cesium or cesium oxide.